1. Field of the Invention
The present invention relates to a crane having a configuration for preventing a mast from collapsing due to operator's operational mistake.
2. Background Art
Heretofore, as a crane such as a crawler crane, there has been known a mast-type crane as disclosed, for example, in JP 2007-290789A. In the mast-type crane, with respect to a front end of a frame of an upper slewing body slewably mounted on a lower propelling body, a base end of a boom is coupled in such a manner as to be pivotable around the front end of the frame, and a base end of a mast for supporting the boom at a high position via a guy line is also coupled in such a manner as to be pivotable around the front end of the frame. Two spreaders are provided, respectively, on a tip end of the mast and on the side of the frame, and a raising and lowering rope is wound around and between the two spreaders. The frame is provided with a raising and lowering winch which is adapted to wind or unwind the raising and lowering rope to allow the boom and the mast to be tilted so as to be raised or lowered.
This type of crane is also equipped with a mast support device (also called, a mast push-up mechanism or a mast raising mechanism) provided on the upper slewing body to push up the mast from a transportation posture where the mast extends approximately horizontally, to a work posture where the mast extends obliquely in such a manner as to become gradually higher toward the tip end thereof. Further, this type of crane is adapted to be capable of performing assembling and disassembling thereof by its own ability, through a suspending work using the mast as substitute for the boom, during the assembling and disassembling of the crane. During the suspending work using the mast, a raising/lowering angle of the mast is limited to a predetermined range by an overload preventing device, in the same manner as that during a normal suspending work, and the mast support device is typically returned to a storage position where it is lowered rearwardly, so as not to hinder the suspending work using the mast. Thus, when the mast is returned from the work posture to the transportation posture after completion of the suspending work using the mast, it is necessary to perform operation in the following sequence:                (1) Moving the mast support device to an overhanging position;        (2) Manually operating a release device, such as a release switch, for releasing the limitation on the raising/lowering angle of the mast by the overload preventing device;        (3) Winding the raising and lowering rope by the raising and lowering winch to tilt the mast to a position where the mast is inclined rearwardly with respect to a vertical state thereof (a state in which the mast raising/lowering angle is 90 degrees), while being supported by the mast support device; and        (4) Returning the mast support device from the overhanging position to the storage position, while supporting the mast, to thereby return the mast to the transportation posture.        
However, when an operator actually returns the mast from the work posture to the transportation posture after completion of the suspending work using the mast, the operator is likely to perform the operation in a different sequence from that described above. In particular, despite the fact that the mast support device is not at the overhanging position, an operator is likely to operate the release device, such as a release switch, without ascertaining the fact, and then operates the raising and lowering winch to wind the raising and lowering rope so as to swingingly move the mast to a position where the mast is inclined rearwardly with respect to the vertical state. This operational mistake gives rise to a problem that the mast falls down rearwardly by its own weight, and resulting shock causes damages to the mast and devices on the upper slewing body.